(1) Field of the invention
The present invention relates to traffic marking paints and more particularly to durable thermoplastic traffic marking paints and methods for their preparation and use.
(2) Description of the Related Art
Traffic marking paints for use on pavement, such as road surfaces, are important elements of modern traffic direction and control systems. A key characteristic of markings and lines of these paints is that they are visible under all environmental conditions. Various improvements have been made over the past several years in traffic paint composition and application methods to maximize the visibility of traffic markings in rain and darkness. The incorporation of reflective elements, such as glass beads, has been one such improvement.
It is known, however, that a layer of water on a road that is sufficient to cover a substantially flat traffic marking stripe can render the reflective elements in the paint ineffective. In order to overcome this problem, several methods and devices have been developed that raise the reflective elements above the level of the water film. Among these devices are stick-on raised reflectors (See, e.g., U.S. Pat. No. 3,332,327); the addition of larger glass beads to the stripe to project above the level of the water film; and the application of traffic marking paint in the form of uniformly or non-uniformly thick profiled stripes (See, e.g., U.S. Pat. No. 5,511,896 to Marcato).
Other important characteristics of traffic marking paints are drying rate and durability. Of course, since it is costly to replace traffic markings, it is desirable that the marking paint is durable. However, some durable paints have long drying times--more than about 20 minutes, for example--and their application can cause significant traffic disruption. Thus, it is important to minimize the time between application of the paint and when vehicles can drive over the paints without smearing, smudging, or removing the wet paint, or transferring the wet paint to cars.
Recently, the use of thermoplastics for traffic marking stripes has been reported to provide several advantages over conventional waterborne, or solvent-based traffic paints. In addition to U.S. Pat. No. 5,511,896 to Marcato, mentioned above, thermoplastic traffic marking paints have been disclosed by Okazaki et al. (U.S. Pat. No. 3,996,645); Tanaka et al. (U.S. Pat. No. 4,324,711); Sato et al. (U.S. Pat. No. 4,297,450); McKenzie (U.S. Pat. No. 4,105,808); Mizui et al. (U.S. Pat. No. 4,086,198); Holmen et al. (U.S. Pat. No. 4,031,048). Raw materials for use in formulating thermoplastic marking materials have been discussed in a brochure titled "Raw materials for thermoplastic pavement striping compounds" (available from Eastman Chemical Company, Kingsport, Tenn.).
Thermoplastic materials are normally applied by melting the thermoplastic composition and applying the molten material to the road surface at a temperature of about 350.degree. F. to about 450.degree. F. When the thermoplastic is applied in a thin coating--below about 0.150" in thickness, for example, the material usually cools quickly enough that it remains substantially where it was applied, without significant spreading or running. Once applied, thermoplastics have proved to be very durable and long-lasting traffic markings.
The unique properties of thermoplastics used for traffic paint applications has led at least one group to propose the creation of raised sections of the traffic stripe in order to improve visibility--especially in wet conditions or at night (U.S. Pat. No. 5,511,896). This patent discloses the application of a thick (0.125" to about 0.250") traffic stripe of a suitable thermoplastic to a road. Before the stripe cools and hardens, an embossing wheel rolls over it and creates an embossed footprint of alternately raised and depressed parts of the stripe. A top coating of reflective beads can be added if desired. The objective is to create raised portions of the stripe that can project above a water film and to create reflective surfaces that are more nearly orthogonal to a driver's normal line of vision. However, problems that can occur when a thermoplastic is applied in a thick stripe is that the material must be maintained at a sufficiently high temperature that it will flow quickly, but this higher temperature can result in thermal degradation of the composition over time as it sits in the applicator tank and the thermoplastic laid in a thick line can spread or run to the extent that the dimensions of the line become irregular and poorly defined. Moreover, any irregular raised profile would have a tendency to slump and puddle.
One way to avoid excessive slumping or puddling is to apply the thermoplastic at a cooler temperature. However, this raises another problem, namely that it is more difficult to flow the cooler and more viscous thermoplastic onto the roadway and to insure that it penetrates and forms a tight adhesive bond with the surface. The speed of application of the composition is slowed and, therefore, application cost is increased.
Another way to control slumping is to add fumed silica to the thermoplastic. Fumed silica is an ingredient that is commonly used for its rheology-adjusting properties, but it is expensive and can increase the cost of the thermoplastic marking composition. Thus, it is preferred that the present traffic stripe composition is free of fumed silica.
It would be desirable, therefore, to provide a thermoplastic traffic marking composition that retained the advantages of durability and speed of application provided by such thermoplastic compositions when used for thin stripes, but that also permitted the application of a thick and/or profiled stripe that retained substantially the profile in which it was applied.